The present invention relates to spectrometry instruments, spectroscopic reflectometers and transmissive spectrophotometers, and relates especially to those spectroscopy instruments which employ a microscope objective and associated imaging optical components for small-spot viewing of a sample having diffractive features to be measured.
Samples with grating-like structures will affect the amplitude and phase of the light they reflect or transmit differently for different incident polarizations. The same is also true for birefringent samples, or stacks of thin films at other than normal incidence. This can be an issue when making measurements with some photometric instruments. In lithography applications, for example, determining the linewidth or profile of diffractive pattern features formed on a semiconductor wafer or photomask may be performed by measuring the normal or near-normal incidence (hereafter collectively referred to as quasi-normal incidence) reflectivity or other optical properties with a small-spot reflectometer or small-spot transmissive spectrophotometer. The spectral reflectivity or transmissivity of the sample being measured will depend to some extent on the degree of polarization of the incident light and the orientation of the wafer.
In some instruments it is possible to orient the sample so that the grating-like structures of the pattern (or the optical axis of a birefringent surface or thin film stack) are presented in a known and consistent direction relative to the instrument""s incident light. Any systematic errors due to polarization can then be minimized during data processing. That is, by carefully characterizing the polarization characteristics of the optics and modeling the effect on a sample""s response at a particular sample orientation relative to the polarized light, the measured data can be processed so as to eliminate the polarization effect provided the sample is measured at the modeled orientation. However, it is not always possible to provide a specified sample orientation to the measuring instrument. Wafer handlers associated with lithography tracks frequently present the samples to the measuring instrument in a consistent but unknown orientation that the measuring instrument itself has no control over. Polishers produce a random sample orientation. Hence, it would be preferable if the instrument""s illumination and collection optics were non-polarizing, so that orienting the wafer would be unnecessary.
In the past, the effect of instrument polarization on measurement results have been only a minor issue that has typically been ignored except in those instruments where polarization itself is the parameter being measured. Polarimeters and ellipsometers deliberately use incident light of known polarization. Also, until recently, spectrometry instruments were not used for measuring linewidth, profile, etc. of grating-like structures.
Unwanted polarization in the optics can be caused by polarizing elements such as tilted fold mirrors, beamsplitters, tilted glass surfaces, prisms, and spectrometer gratings. (In this context xe2x80x9cpolarizingxe2x80x9d can mean partially polarizing or in some way affecting the polarization state.) One prior solution has been to reduce the polarization effect of instrument components by carefully arranging the planes of incidence of the tilted components in the system, so that for every such tilted component the instrument also has a similar component tilted in the perpendicular plane to cancel the polarization effect of the first. This use of component pairs requires more room for the optics, so that it cannot be used when a compact system is needed. The pairing technique cannot be used to alleviate the polarization effect in the spectrometer component of the system. In Zeiss monolithic spectrometers, among others, light is coupled with a fiberoptic bundle that scrambles the polarization.
Depolarizers of several types are known. Fiber depolarizers cannot be used in the imaging path because they would also scramble information about the image. Wedge depolarizers, comprising a birefringent wedge plate and an index-matched non-birefringent plate, need to be properly oriented to the polarization of the light to be depolarized. Because they produce a laterally offset double image, they are not well suited for imaging systems. Lyot depolarizers, comprising two non-wedge-shaped birefringent plates with their axes at 45xe2x80x94 to each other, are commercially available. They have previously been used in imaging spectroradiometers and spectropolarimeters for telescopes, for example on a satellite observing backscattered radiation from the earth to monitor atmospheric ozone depletion. In contrast to fiber and wedge depolarizers, Lyot depolarizers are image-preserving, and are therefore suitable for imaging systems.
An object of the present invention is to provide a small-spot spectrometry instrument with pattern viewing capability for measuring grating-like or other diffractive pattern structures on semiconductor wafers, photomasks, and the like, wherein the instrument""s polarization effects on linewidth, profile, erosion and similar feature measurements are minimized.
The object has been met by a small-spot imaging, spectrometry instrument in which a polarization-scrambling element such as a Lyot depolarizer is incorporated between the beamsplitter and the microscope objective. The beamsplitter is the last significant polarizing element in the illumination path prior to the sample. Preferably the Lyot depolarizer is placed in a collimated portion of the light path to avoid creating a double image offset in focus. The Lyot depolarizer does not vary the polarization spatially as wedge depolarizers do. Rather, the Lyot depolarizer varies the polarization with wavelength. The sinusoidally perturbed spectrum that results can be removed by data processing techniques. If the depolarizer is made thick enough or made from a highlybirefringent material, such as calcite or alpha barium borate, than the sinusoidal perturbation may be much narrower than the wavelength resolution of the instrument. In this case the perturbation would not be detectable and no processing would be required to remove it. The only disadvantage of using calcite for the depolarizer material is that it does not transmit as much UV light as quartz. A disadvantage of alpha barium borate is its high cost. When both the illuminating and collected light pass through the same depolarizer, there is a preferred orientation for the depolarizer.